Electrode for use in a capacitor

ABSTRACT

A deposition process for coating a substrate with a ruthenium-containing oxide material, or a precursor thereof, is described. Preferably, the reagent solution is substantially devoid of a halide, such as chlorine. The coated substrate is useful as an electrode in a capacitor.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation-in-part of U.S. application Ser. No. 09/808,582, now U.S. Pat. No. 6,468,605 to Shah et al., which is a divisional of U.S. Pat. No. 6,224,985 to Shah et al., which is a divisional of U.S. Pat. No. 5,920,455 to Shah et al.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention generally relates to coating a substrate with a ruthenium-containing oxide compound. More particularly, the present invention relates to a metallic foil provided with a ruthenium oxide or a ruthenium-containing oxide for use in a capacitor, and the like.

[0004] 2. Prior Art

[0005] Whether an anode or a cathode in an electrochemical capacitor or the cathode in an electrolytic capacitor, a capacitor electrode generally includes a substrate of a conductive metal such as titanium or tantalum provided with a ruthenium oxide coating or a ruthenium-containing oxide coating. In the case of a ruthenium oxide cathode, the coating is formed on the substrate by dissolving a ruthenium-containing halide precursor in a solvent. Typically used precursors include ruthenium(III) chloride hydrate and ammonium hexachlororuthenium(III).

[0006] The problem with ruthenium halide precursors is that when the resulting electrode is coupled with a tantalum anode, the halide, for example chlorine, leaches into the electrolyte and slowly attacks the tantalum oxide (TaO₅) of the anode to form tantalum chloride (TaCl₃). And, chlorine contaminated tantalum oxide has a higher leakage current and lower capacitance than a tantalum oxide coated component which is not so contaminated. Higher leakage currents and lower capacitance result in longer charge times and lower charge/discharge efficiency, and for that reason is undesirable, especially when the electrical energy storage device is a capacitor.

SUMMARY OF THE INVENTION

[0007] The present invention describes the deposition of a ruthenium oxide compound or a ruthenium-containing oxide compound, or a precursor thereof, onto a conductive substrate. The reagent solution is substantially devoid of halides to prevent contamination of the counter electrode when the coated substrate is used as an electrode in an electrical energy storage device, such as a capacitor. The substrate is preferably heated during the deposition process and supports the coating as it is solidified or, in the case of a precursor, converted to a solidified product compound.

[0008] These and other aspects of the present invention will become more apparent to those skilled in the art by reference to the following description and the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a schematic of a unipolar electrode configuration for use in an electrochemical capacitor.

[0010]FIG. 2 is a schematic of a bipolar electrode configuration for use in an electrochemical capacitor.

[0011]FIG. 3 is a schematic of a hybrid capacitor according to the present invention.

[0012]FIG. 4 is a schematic of a spirally wound configuration for use in an electrochemical capacitor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The first step in the process includes providing a solution of reagents. The reagent solution preferably contains ions in substantially the ratio needed to form the desired coating. These ions are preferably available in solution in water-soluble form such as in water-soluble salts of nitrates, acetates, and sulfates. However, salts including nitrates, acetates and sulfates that are soluble in other solvents such as organic and inorganic solvents may be used.

[0014] The substrate preferably consists of a conductive metal such as titanium, molybdenum, tantalum, niobium, cobalt, nickel, stainless steel, tungsten, platinum, palladium, gold, silver, copper, chromium, vanadium, aluminum, zirconium, hafnium, zinc and iron, and the like, and mixtures and alloys thereof.

[0015] Regardless of the material of substrate, ruthenium-containing oxide coatings rely mostly upon mechanical bonding to the substrate surface. It is, therefore, critical that the substrate surface to be coated is properly prepared to ensure coating quality. For one, substrate surface cleanliness is very important in all coating systems, especially in ultrasonically deposited spray coatings. In that respect, it is required that the substrate surface remain uncontaminated by lubricants from handling equipment or body oils from hands and the like. Substrate cleaning includes chemical means such as conventional degreasing treatments using aqueous and non-aqueous solutions, as are well known to those skilled in the art. Plasma cleaning is also contemplated by the scope of the present invention.

[0016] It is further contemplated by the scope of the present invention that, if desired, the electrical conductivity of the substrate is improved prior to coating. Metal and metal alloys have a native oxide present on their surface. This is a resistive layer and hence, if the material is to be used as a substrate for a capacitor electrode, the oxide is preferably removed or made electrically conductive prior to deposition of a pseudocapacitive coating thereon. In order to improve the electrical conductivity of the substrate, various techniques can be employed. One is shown and described in U.S. Pat. No. 5,098,485 to Evans, the disclosure of which is hereby incorporated by reference. A preferred method for improving the conductivity of the substrate includes depositing a minor amount of a metal or metals from Groups IA, IVA and VIIIA of the Periodic Table of Elements onto the substrate. Aluminum, manganese, nickel and copper are also suitable for this purpose. The deposited metal is then “intermixed” with the substrate material by, for example, a high-energy ion beam or a laser beam directed towards the deposited surface. These substrate-treating processes are performed at relatively low temperatures to prevent substrate degradation and deformation. Additionally, these treating processes can be used to passivate the substrate from further chemical reaction while still providing adequate electrical conductivity. For additional disclosure regarding improving the electrical conductivity of the substrate prior to deposition, reference is made to U.S. patent application Ser. No. 08/847,946 entitled “Method of Improving Electrical Conductivity of Metals, Metal Alloys and Metal Oxides”, which is assigned to the present invention and incorporated herein by reference.

[0017] Surface roughness is another critical factor to consider when properly applying a ruthenium-containing oxide coating. The substrate may be roughened by chemical means, for example, by contacting the substrate with hydrofluoric acid and/or hydrochloric acid containing ammonium bromide and methanol and the like, by plasma etching, and by mechanical means such as scraping, machining, wire brushing, rough threading, grit blasting, a combination of rough threading then grit blasting and abrading such as by contacting the substrate with Scotch-Brite™ abrasive sheets manufactured by 3M.

[0018] The reagent solution preferably contains ions in substantially the stoichiometric ratio needed to form the desired coating. The ions are present in the reagent solution as either a water-soluble salt or as one that is soluble in organic and inorganic solvents such as isopropyl alcohol and nitric acid, and mixtures thereof. A preferred reagent precursor for a ruthenium oxide coating is a ruthenium nitrate, ruthenium acetate, or ruthenium sulfate, or an organic salt. In that respect, suitable precursors include the water-soluble salts of ruthenium(III) nitrosyl nitrate, nitrosyl ruthenium(III) acetate, and nitrosyl ruthenium(III) sulfate. Alcohol soluble salts include nitrosyl ruthenium(III) acetate, nitrosyl ruthenium(III) sulfate and ruthenium(III) 2,4-pentanedionate. Ruthenium oxide can be coated on the substrate without having to go through the precursor step.

[0019] The porous coating may also include a second or more metals. The second metal is in the form of an oxide, a nitrate, acetate or a sulfate, and is not essential to the intended use of the coated foil as a capacitor electrode, and the like. The second metal is selected from one or more of the group consisting of tantalum, titanium, nickel, iridium, platinum, palladium, gold, silver, cobalt, molybdenum, manganese, tungsten, iron, zirconium, hafnium, rhodium, vanadium, osmium and niobium. In a preferred embodiment of the invention, the porous coating product includes oxides of ruthenium and tantalum.

[0020] The reagent solution is preferably at a concentration of from about 0.01 to about 1,000 grams of the reagent compounds per liter of the reagent solution. In one embodiment of the present invention, it is preferred that the reagent solution has a concentration of from about 1 to about 300 grams per liter and, more preferably, from about 5 to about 60 grams per liter.

[0021] Suitable methods for contacting the substrate with the reagent solution include dipping, painting, doctor-blading, pressurized air atomization spraying, or aerosolized spraying of the material onto the substrate. Sol-gel deposition is another suitable coating method. For additional disclosure regarding the deposition of a ruthenium-containing material on a substrate as an aerosolize spray, reference is made to U.S. Pat. Nos. 5,894,403 to Shah et al., 5,920,455 to Shah et al., 6,224,985 to Shah et al., and 6,468,605 to Shah et al., all of which are assigned to the assignee of the present invention and incorporated herein by reference.

[0022] In one embodiment of the present invention, regardless of the deposition process the substrate is heated at a temperature sufficient to instantaneously evaporate or otherwise drive off the solvent from the deposited ruthenium-containing reagent solution. The coated substrate is subsequently heated to convert the precursor to the desired ruthenium oxide. Thus, as the substrate is being coated, it is heated to a temperature sufficient to drive off or otherwise evaporate the solvent material. Preferably the solvent is evaporated from the substrate almost instantaneously with contact of the reagent solution, resulting in the deposition of a relatively thin film oxide coating. In the case of the solvent consisting of a nonaqueous solution, for example, isopropyl alcohol and nitric acid, the substrate is preferably heated to a temperature of from about 70° C. to about 95° C., preferably about 85° C. This temperature is considered sufficient to instantaneously evaporate the solvent from the deposited reagent mixture. In the case of an aqueous solution, the substrate is heated to a temperature of from about 100° C. to about 300° C. to instantaneously evaporate the water from the deposited reagent mixture.

[0023] The reagent material is then heated to a temperature sufficient to convert the deposited precursor to a highly porous, high surface area ruthenium-containing oxide. For oxides, typical heating times range from about one-half hour to about six hours. More preferably, after spraying and solvent removal, the oxide precursor coated substrate is heated to a temperature of about 100° C. to 300° C., preferably about 250° C. for about one hour, followed by a further heating at a temperature of about 250° C. to 400° C., preferably about 300° C. for about two hours. A further heating at a temperature of about 100° C. to about 500° C., preferably about 400° C. for about two hours and fifteen minutes immediately follows this. While this three step heating protocol is preferred for converting the above described precursors to a ruthenium oxide, it is contemplated by the scope of the present invention that a two step or a four step or more heating protocol can also be used. Alternatively, the temperature is slowly and steadily ramped up, for example, at about 1° C./minute, preferably about 6° C./min. until the temperature reaches about 400° C. where it is maintained for a time sufficient to convert the precursor to its final product material.

[0024] In another embodiment, the substrate is maintained at a temperature sufficient to, for all intents and purposes, instantaneously convert the precursor to a porous, high surface area ruthenium-containing oxide coating on the substrate. More particularly, as the precursor reagent solution is deposited onto the substrate, the substrate is at a temperature of about 100° C. to about 500° C., preferably at least about 200° C., and more preferably about 350° C. to instantaneously convert the precursor to the oxide coating.

[0025] After deposition and conversion of the precursor to the ruthenium-containing oxide compound, whether it is instantaneous or otherwise, the substrate is ramped down or cooled to ambient temperature, maintained at the heated deposition temperature to enhanced bonding strength, or varied according to a specific profile. In general, it is preferred to conduct the heating steps while contacting the substrate with air or an oxygen-containing gas.

[0026] It is preferred that the resulting ruthenium-containing oxide coating have a thickness of from about a hundred Angstroms to about 0.1 millimeters or more. The porous coating has an internal surface area of about 10 m²/gram to about 1,500 m²/gram. In general, the thickness of the substrate is typically in the range of about 0.001 millimeters to about 2 millimeter and preferably about 0.1 millimeters. The thickness of the coating on the substrate is determined by means well known to those skilled in the art. Also, the temperature of the substrate affects the crystal structure and coating adhesion strength. Higher heating temperatures result in increased intermingling of the deposited reagent materials with the surface ions of the substrate, thereby affecting bonding strength.

[0027] An ultrasonically coated substrate according to the present invention is useful as an electrode in various types of electrochemical capacitors including unipolar and bipolar designs, and capacitors having a spirally wound configuration. For example, in FIG. 1 there is shown a schematic representation of a typical unipolar electrochemical capacitor 10 having spaced apart electrodes 12 and 14. One of the electrodes, for example, electrode 12, serves as the cathode electrode and comprises a coating 16A of a ruthenium-containing oxide material provided on a conductive plate 18A according to the present invention. For example, a porous ruthenium oxide film is provided on a tantalum plate 18A. The relative thicknesses of the plate 18A and the coating 16A thereon are distorted for illustrative purposes. As previously described, the plate is about 0.01 millimeters to about 1 millimeter in thickness and the ruthenium-containing oxide coating 16A is in the range of about a few hundred Angstroms to about 0.1 millimeters thick. The other electrode 14 serves as the anode and is of a ruthenium-containing oxide material 16B contacted to a conductive substrate 18B, as in electrode 12.

[0028] An ion permeable membrane 20 serving as a separator separates the cathode electrode 12 and the anode electrode 14 from each other. The electrodes 12 and 14 are maintained in the spaced apart relationship shown by opposed insulating members 22 and 24 such as of an elastomeric material contacting end portions of the plates 18A, 18B. The end plate portions typically are not coated with a ruthenium-containing oxide material. An electrolyte (not shown), which may be any of the conventional electrolytes used in electrolytic capacitors, such as a solution of sulfuric acid, potassium hydroxide, or an ammonium salt, is provided between and in contact with the cathode and anode electrodes 12 and 14. Leads (not shown) are attached to the electrodes 12 and 14 before, during, or after assembly of the capacitor. The thusly-constructed unipolar capacitor configuration is housed in a suitable casing (not shown), or the conductive plates along with the insulating members can serve as the capacitor housing.

[0029]FIG. 2 is a schematic representation of a typical bipolar electrochemical capacitor 30 comprising a plurality of capacitor units 32 arranged and interconnected serially. Each unit 32 includes a bipolar conductive substrate 34 having ruthenium-containing oxide coatings 36 and 38 provided on its opposite sides. Again, the thickness of the porous coatings 36 and 38 is distorted for illustrative purposes. The units 32 are then assembled into the bipolar capacitor configuration on opposite sides of an intermediate separator 40. Elastomeric insulating members 42 and 44 are provided to maintain the units 32 in their spaced apart relationship. As shown in the dashed lines, a plurality of individual electrochemical capacitor units 32 is interconnected in series to provide the bipolar configuration. The serial arrangement of units 32 is completed at the terminal ends thereof by end plates (not shown), as is well known to those skilled in the art. An electrolyte 44 is provided between and in contact with the coatings 36, 38 of the capacitor 30.

[0030]FIG. 3 shows a schematic representation of an electrolytic capacitor 50 having spaced apart cathode electrodes 52, 54, each comprising a ruthenium-containing oxide coating 52A, 54A provided on a conductive plate 52B, 54B according to the present invention. The counter electrode or anode 56 is intermediate the cathodes 52, 54 with separators 58, 60 preventing contact between the electrodes. The anode 56 is of a conventional sintered metal, preferably in a porous form. Suitable anode metals are selected from the group consisting of titanium, aluminum, niobium, zirconium, hafnium, tungsten, molybdenum, vanadium, silicon, germanium and tantalum contacted to a terminal pin 62. Insulating members 64, 66 contacting end portions of the cathode plates, complete the hybrid capacitor 50. While not shown, an electrolyte is provided to activate the electrodes 52, 54 and 56.

[0031]FIG. 4 is a schematic drawing of another embodiment of a jellyroll-configured capacitor 70, which can be manufactured by any one of the processes according to the present invention. Capacitor 70 has a plurality of capacitor units 72, each comprising a conductive substrate provided with a ruthenium-containing oxide coating 74, 76 on the opposed sides thereof. The coatings are separated from immediately adjacent cells by an intermediate separator 78. This structure is then wound in a jellyroll fashion and housed in a suitable casing. Leads are contacted to the anode and cathode electrodes and the capacitor is activated by an electrolyte in the customary manner.

[0032] The following example describes the manner and process of coating a substrate according to the present invention, and they set forth the best mode contemplated by the inventors of carrying out the invention, but they are not to be construed as limiting.

EXAMPLE I

[0033] A precursor solution was prepared by dissolving 2.72 grams of ruthenium nitrosyl nitrate in a solvent that consisted of 100 cc of deionized water. If needed, a minor amount, i.e. about 5 cc of nitric acid is used to completely solubilize the precursor. The solution was stirred until the ruthenium nitrosyl nitrate was completely dissolved.

[0034] A tantalum foil, 0.002″ thick, which was cleaned with appropriate cleaning solutions and mounted on the temperature controlled substrate holder, served as the substrate. The foil was heated to a temperature of 350° C. As the reagent solution was deposited on the heated substrate, the solvent evaporated and a ruthenium nitrosyl nitrate film was created on the surface of the foil. The foil temperature of 350° C. was sufficient to convert the ruthenium nitrosyl nitrate to ruthenium oxide substantially instantaneously as the deposition occurred. On completion of the deposition process, the film was allowed to remain on the heater block for half an hour in order to ensure that the entire nitrate had converted to the oxide.

[0035] It is appreciated that various modifications to the inventive concepts described herein may be apparent to those skilled in the art without departing from the spirit and the scope of the present invention defined by the hereinafter appended claims. 

What is claimed is:
 1. An electrode component, which comprises: a) a substrate having a surface to be coated; and b) a ruthenium-containing oxide compound coated thereon, wherein the coating is characterized as having been formed from a solution comprising a solvent having a precursor of the ruthenium-containing oxide compound dissolved therein and contacted to the substrate, the substrate having been heated to evaporate the solvent and convert the precursor to the ruthenium-containing oxide compound coated on the substrate surface, and wherein the precursor is substantially devoid of a halide.
 2. The electrode component of claim 1 wherein the precursor is selected from the group consisting of ruthenium(III) nitrosyl nitrate, nitrosyl ruthenium(III) acetate, nitrosyl ruthenium(III) sulfate, ruthenium(III) 2,4-pentanedionate, and mixtures thereof.
 3. A method for providing an electrode component, comprising the steps of: a) providing a substrate having a surface to be coated; b) providing a solution comprising a solvent having a precursor for a ruthenium-containing oxide compound dissolved therein, wherein the precursor is substantially devoid of a halide; and c) contacting the substrate with the solution, wherein the substrate is heated to a temperature sufficient to at least partially evaporate the solvent and to instantaneously convert at least some of the precursor to a ruthenium-containing oxide compound coated on the substrate surface.
 4. The method of claim 3 including selecting the precursor from the group consisting of ruthenium(III) nitrosyl nitrate, nitrosyl ruthenium(III) acetate, nitrosyl ruthenium(III) sulfate, ruthenium(III) 2,4-pentanedionate, and mixtures thereof.
 5. The method of claim 3 including contacting the solution to the substrate by one of dipping, painting, doctor-blading, pressurized air atomization spraying, aerosolized spraying, and sol-gel deposition.
 6. A method for providing an electrode component, comprising the steps of: a) providing a substrate having a surface to be coated; b) providing a solution comprising either an aqueous solvent or an alcohol solvent, or a mixture thereof, having a precursor of a ruthenium-containing oxide compound, wherein the precursor is substantially devoid of a halide; c) contacting the substrate with the solution; and d) heating the substrate to evaporate the solvent and convert the precursor, thereby forming a coating of the ruthenium-containing oxide compound on the substrate surface.
 7. The method of claim 6 including selecting the precursor of the ruthenium-containing oxide compound from the group consisting of a nitrate, acetate, sulfate, organic salt, and mixtures thereof.
 8. The method of claim 6 including selecting the precursor from the group consisting of ruthenium(III) nitrosyl nitrate, nitrosyl ruthenium(III) acetate, nitrosyl ruthenium(III) sulfate, ruthenium(III) 2,4-pentanedionate, and mixtures thereof.
 9. The method of claim 6 including contacting the solution to the substrate by one of dipping, painting, doctor-blading, pressurized air atomization spraying, aerosolized spraying, and sol-gel deposition.
 10. The method of claim 6 wherein the substrate is heated to a temperature of at least about 100° C. for the aqueous solution and at least about 70° C. for an alcohol solution.
 11. The method of claim 6 including providing the coating having a majority of its particles having diameters of less than about 10 microns.
 12. The method of claim 6 including providing an internal surface area of the coating of about 10 m²/gram to about 1,500 m²/gram.
 13. The method of claim 6 including providing the coating having a thickness of about a hundred Angstroms to about 0.1 millimeters.
 14. The method of claim 6 including providing a second metal in the solution.
 15. The method of claim 14 including selecting the second metal from the group consisting of tantalum, titanium, nickel, iridium, platinum, palladium, gold, silver, cobalt, molybdenum, ruthenium, manganese, tungsten, iron, zirconium, hafnium, rhodium, vanadium, osmium, niobium, and mixtures thereof.
 16. The method of claim 6 including selecting the substrate from the group consisting of tantalum, titanium, nickel, molybdenum, niobium, cobalt, stainless steel, tungsten, platinum, palladium, gold, silver, copper, chromium, vanadium, aluminum, zirconium, hafnium, zinc, iron, and mixtures thereof.
 17. The method of claim 6 including increasing the surface area of the substrate surface prior to contacting the solution to the substrate.
 18. The method of claim 17 including increasing the substrate surface area by at least one of the group consisting of rough threading, grit blasting, scraping, plasma etching, abrading, wire brushing, and acid contact.
 19. The method of claim 6 including increasing the electrical conductivity of the surface of the substrate prior to contacting the solution thereto.
 20. The method of claim 6 including providing the substrate having a thickness of about 0.001 to about 2 millimeters. 